Conventional light boxes for exposing a light-sensitive sheet to a master sheet commonly include an impervious rubber blanket that overlays the stack of sheets to form an air-tight seal against a glass support plate during exposure to light through the glass. A vacuum source evacuates air from between the layers of sheets, glass and blanket with the result that the force of air pressure acting upon the outer surface of the blanket holds the two sheets tightly together in precise registration.
Various mechanisms have been used to support and deploy such rubber blankets during the operating phases when the sheets are being positioned or removed before and after exposure to light. One known scheme supports the blanket above the glass plate on a movable lid that is hinged along the rear edge of the glass plate. Another known scheme allows a rolled blanket to unroll over the glass plate and sheets to be registered that are placed thereon, and then to re-roll the blanket back to a location near the front working edge of the glass plate after the exposure phase. Residual air remaining between the sheets and under the blanket is evacuated through holes in the glass plate to allow the force of air pressure acting upon the outer surface of the unrolled blanket to hold the master and copy sheets in precise registration.
One disadvantage of these known schemes is that the light box becomes directionally limited in that it becomes awkward to service or inspect registration of sheets on the glass plate from any edge. In addition, cleaning of the glass plate is impeded by the structure required to support or house the blanket.